A project to evaluate the CD45 alternative isoform, B220, expression on lymphocytes has been initiated. This is being done in healthy subjects and patients with autoimmune lymphoproliferative syndrome (ALPS) as well as other patients with lymphocyte disorders. Preliminary data suggest that this isoform is expressed at different levels in certain lymphocyte disorders and may serve as a characteristic finding in subtypes of ALPS. In addition, it appears to distinguish a subset of memory B cells and also is expressed on ono-isotype switched B cells that do not have a memory phenotype.